1. Field of Invention
The present invention relates generally to magnetic sensing devices, and more particularly to the arrangement of magnetic sensor units in a magnetic sensing device.
2. Description of Related Art
Magnetic sensing devices facilitate the measurement of a magnetic field (i.e. one or more magnetic fields) for a variety of applications by using one or more magnetic sensor units to sense the magnetic field, and to provide output signals that represent the magnetic field. Navigation applications that determine a heading determination are popular applications for magnetic sensing devices. A heading determination may indicate a direction, such as North or North-East. Other applications for magnetic sensing devices, such as proximity detection, are also possible.
The one or more magnetic sensor units in a magnetic sensing device may be arranged in a manner that provides sensing of particular components of a magnetic field. For example, a first magnetic sensor unit may be arranged to sense a component of a magnetic field in a direction defined as the x-axis direction, and a second magnetic sensor unit may be arranged to sense a component of the magnetic field in a direction defined as the y-axis direction. In this example, the magnetic sensing device could have a first output to provide an output signal that represents components of the magnetic field in the x-axis direction and a second output to provide an output signal that represents components of the magnetic field in the y-axis direction.
A wide variety of magnetic sensor unit types are available such as reed switches, variable reluctance sensors, flux-gate magnetometers, magneto-inductor sensors, spin-tunnel device sensors, and Hall-Effect sensors. Another magnetic sensor unit type is a magnetic sensor unit that comprises magnetoresistive material. Examples of magnetic sensors comprising magnetoresistive material include giant magneto-resistive sensors and giant magneto-impedance sensors. Other examples are also possible.
Magnetoresistive material is a material with a variable resistance value that varies depending in part on a magnetic field in proximity to the magnetoresistive material. The sensitivity of magnetoresistive material to change its resistance value when exposed to a magnetic field depends in part on the characteristics of a particular magnetoresistive material. Common magnetoresistive materials include anisotropic magnetoresistive (AMR) materials and giant magnetoresistive (GMR) materials which are both described in U.S. Pat. No. 5,569,544 and colossal magnetoresistive (CMR) materials described in U.S. Pat. No. 5,982,178.
One type of AMR material is a nickel-iron material known as Permalloy. AMR-type magnetic sensor units may include thin films of Permalloy deposited on a silicon wafer and patterned as a resistor. Multiple resistors made of Permalloy may be coupled together to form an electrical circuit. The electrical circuit could take the form of a bridge configuration, such as a Wheatstone bridge configuration.
FIG. 1 illustrates a magnetic sensor unit 10 that includes a first resistor 12, a second resistor 14, a third resistor 16, and a fourth resistor 18 coupled together in a Wheatstone bridge configuration. First ends of the first and second resistors 12 and 14 are connected to a common power source 20, such as a voltage source supplying a positive voltage. First ends of the third and fourth resistors 16 and 18 are connected to a common ground source 22. Second ends of the first and third resistors 12 and 16 are connected to a first input of an amplifier 24 and second ends of the second and fourth resistors 14 and 18 are connected to a second input of the amplifier 24. The amplifier 24 produces an output (V out) which is an amplified differential signal.
The resistance values of the first, second, third, and fourth resistors 12, 14, 16, 18 are typically chosen to be equivalent resistance values. The first, second, third, and fourth resistors 12, 14, 16, 18 could be made with a magnetoresistive material.
During fabrication of AMR-type magnetic sensor units, the AMR magnetoresistive material is deposited on a silicon substrate in the presence of a strong magnetic field. This strong magnetic field sets a magnetization vector in the AMR magnetoresistive material resistor to be parallel to the length of the resistor by aligning the magnetic domains of the AMR magnetoresistive material in the same direction. Magnetic domains are clusters of atoms within the AMR magnetoresistive material with their magnetic moment pointing in the same direction.
FIG. 2 illustrates a plan view of a strip of AMR magnetoresistive material 40 having a magnetization vector 42 in a first direction. A current 43 could pass through the strip 40, from a first side 44 of strip 40 to a second side 45 of strip 40, at an angle 46 in relation to the magnetization vector 42 when no magnetic field is applied to the strip 40, by placing conductive straps, such as conductive straps 47, 48, across the strip 40 at an angle 49. The angle between the current 43 and the magnetization vector 42 occurs, in part, because of the angle 49 formed by the conductive straps 47, 48 placed across strip 40. Angle 46 is preferably about 45° when there is no magnetic field applied to the strip 40.
For the current 43 to pass through strip 40 at the preferred angle 46 of about 45°, angle 49 is also 45°. Angle 49 is formed between each of the conductive straps 47, 48 and a side 50 of the magnetoresistive strip 40 as shown in FIGS. 2 and 3. The current 43 passes through strip 40 in a direction that is substantially perpendicular to the conductive straps 47, 48. The number of conductive straps placed across strip 40 could be greater than or less than the two straps shown in FIGS. 2 and 3.
Conductive strap 47 is an example of one of the conductive straps across strip 40. U.S. Pat. No. 4,847,584 describes the placement of conductive straps on magnetoresistive material.
FIG. 3 illustrates a plan view of strip 40 when a magnetic field 52 is applied normal (perpendicular) to a side 50 of strip 40. The current 43 continues to pass through the strip 40 in the same direction as the current 43 in FIG. 2 due to the orientation of the conductive strap 48 and others conductive straps similarly placed across the strip 40. However, the magnetic field 52 causes the magnetization vector 54 to rotate.
The rotation of the magnetization vector 54, in this case, causes the size of angle 56 formed between the current 43 and the magnetization vector 54 to decrease with respect to the size of angle 46. As the size of angle 56 decreases the resistance of the strip 40 increases. Other arrangements of the strip 40 and the conductive straps 47, 48 could cause the size of angle 56 to increase which decreases the resistance of the strip 40.
Magnetic sensing devices are available in a variety of one-axis and two-axis configurations. The number of axes in a magnetic sensing device refers to the number of sensitive axes or sensing directions for measuring a magnetic field. Magnetic sensing devices with more than one axis typically arrange the multiple axes to be mutually orthogonal. Some forms of three-axis magnetic sensing devices are available but not in the integrated form as described below.